The invention is particularly applicable for inductively heating the exhaust valve seats or valve seat inserts of internal combustion engines and will be described with reference thereto; however, the invention, in its broader application, may be used in other heat treating applications wherein accurate positioning of a heating device is required to assure uniform heating of and properties for the surface to be treated.
To provide the required extended wear characteristics for the poppet valve-valve seat interface, it has become commonplace to quench harden the generally conical valve seat area. The necessary hardened surface for extended wear can be provided by accurately locating an inductor coil adjacent the seat, energizing the coil with a high frequency current to raise the temperature to the desired temperature, followed by rapid quenching.
To achieve the required heating uniformity and temperature control, it is necessary that the inductor be accurately located with respect to the conical valve surface, with an inductive air gap in the order of 0.030 to 0.050 inches. This accuracy is difficult to attain in mass production unless the inductor positioning apparatus can account and compensate for the manufacturing variations in the radial and axial location of the valve seat.
Various approaches have been taken to provide the required inductor positioning accuracy. U.S. Pat. No. Re. 29,046, illustrates a machine wherein the individual inductor assemblies are allowed to radially float with respect to the valve seat as they axially approach the valve seat area. The inductor assemblies carry a centering nose which enters a valve bore coaxial with the valve seat. This radially locates the inductor coaxial with the valve seat, notwithstanding manufacturing variances in the location of the valve axis. Other approaches for achieving this radial alignment feature are illustrated in U.S. Pat. Nos. 4,266,109; 3,837,934; 3,777,096; 3,761,669; 3,743,809 and 3,737,612. Having achieved concentric coaxial alignment with the valve seat, it is also necessary to accurately establish the axial location of the inductor with respect to the valve seat inasmuch as this dimension can vary from valve to valve and engine to engine. This correct positioning must be attained to establish the required inductive air gap to optimumly heat at a controlled temperature, and with regard to the simultaneous heating of multiple valve seats, an inductive current balance among the various inductors.
One successful approach, as illustrated in the afore-mentioned U.S. Pat. No. Re. 29,046, has been to individually seat the concentrically located inductor with the valve seat against a spring bias, axially lock the inductor to its frame and then axially withdraw the inductor a predetermined axial distance corresponding to the desired inductive gap. This repetitively provides accurate inductor positioning notwithstanding axial and/or radial variation in the location of the valve seat surface.